PTL 1 discloses a seat in which in order to allow an occupant to easily get on/off a vehicle, a seat back of the seat positioned in the vicinity of a door is folded forward and a forward slide of the seat expands a space behind the seat. The seat described in Patent Document 1 is configured to unlock, when the seat back is folded forward beyond a predetermined angle, a slide lock of the seat, and to slide forward. In the present description, a state where a seat back is sufficiently folded forward is called a forward folding state, and a state where the seat in the forward folding state is then slid forward is called a walk-in state.
Moreover, PTL 2 discloses, in order to improve a ride quality of an occupant, providing side support parts for supporting the occupant from left and right sides, to a seat cushion and a seat back. Such a configuration can restrict the occupant being shifted in the transverse direction when a vehicle goes around a curve.